1. Field of the Invention
This invention relates to electrically conductive transparent panels and to bus bars which make electrical contact with them. These transparent panels can take the form of window glazings, particularly vehicle windshields and the like. The electrical bus bars can be used to allow a powering current to be delivered to the panel to heat the panel such as for deicing or defogging a vehicle windshield. They also can be used to permit an electromagnetic signal, such as a radio signal, detected by the panel to be electrically transmitted from the panel.
2. Background Information
There are numerous applications where it is desired to have a transparent panel containing an electrically conductive element which is connected to an electrical circuit. Three very common applications of this sort are 1. vehicle window glazings which have conductive elements for providing electric heat to the windows to melt ice or defog; 2. vehicle window glazings which have conductive elements as antennas for radios and the like; and 3. architectural and vehicular window glazings which provide electromagnetic radiation shielding. There is a growing interest in these electrically conductive layer-carrying glazing products in the automotive field. There, the needs for increased automotive efficiency and streamlining call for more gently sloped, larger area windows and "in glass" antennas. There is also a need to provide deicing and defogging of these window surfaces, particularly because modern cars are expected to operate with their windows closed for safety and efficiency reasons.
One approach to providing conductive elements in transparent panels, such as window glazings, has been to apply fine wires to the surface of the panel or to imbed such wires in the panels or between multiple panels and to make an electrical connection to the wires. A second approach has been to apply over at least a substantial portion of the transparent surface a thin film coating which is electrically conductive and which, because of its thinness and/or its chemical make up, is itself substantially transparent. In this case, an electrical connection must be made to the conductive thin film. It is this second approach to which this invention relates.
The structures which effect the electrical connection to the thin film conductive elements are often referred to as "bus bars". They are often patterned to achieve proper current distribution or to focus heating into certain regions in the case of heated windows and the like. Several methods and materials have been used heretofore to make these electrical connections. One common type of connection is a silk-screened layer of conductive ink or paint such as a silver epoxy which is thermally cured. Another conventional type of bus bar employs conductive tapes and foils in contact with the conductive layer. In both of these settings it is common to laminate the resulting conductive layer-bus bar assembly into a glazing structure with poly(vinylbutyral) (PVB).
Both of these methods gave poor results. After lamination, the bus to bus resistance increases significantly. It is not clear if the increase in bus to bus resistance is a result of degradation of the electrical contact between the bus bar and the transparent conductor or if it is a result of cracking in the transparent conductor in the vicinity of the bus bar which might be caused by physical stress caused by uneven pressure applied through the relatively thick bus bar to the film during lamination. Whatever the failure mechanism of these conventional assemblies, when bus bars of the present invention were used, the bus to bus resistance did not increase signigicantly during the lamination cycle.
Conductive enamel frits have been tried as electrical conductors. The materials are brittle and require high temperature firing which can disturb delicate conductive elements. Electrodeposition of conductive connectors has also been used but has the disadvantage of employing wet chemistry which is typically different than and not particularly compatible with the other processes employed in the production of the conductive thin films.
There is a substantial body of information available concerning the application of electrically conductive layers to windows and other transparent panels. Representative descriptions can be found in the following United States patents:
U.S. Pat. No. 5,089,687 to PPG (Feb. 18, 1992) PA0 U.S. Pat. No. 5,448,037 to Mitsui Toatsu Chemicals Inc. (Sep. 5, 1995) PA0 U.S. Pat. No. 5,070,230 to Ashai Glass (Dec. 3, 1991) PA0 U.S. Pat. No. 4,593,175 to PPG (Jun. 3, 1986) PA0 U.S. Pat. No. 4,284,677 to Libbey Owens Ford (Aug. 18, 1981) PA0 U.S. Pat. No. 3,833,451 to PPG (Sep. 3, 1974) PA0 U.S. Pat. No. 3,790,752 to Ford Motor Company (Feb. 5, 1974) PA0 U.S. Pat. No. 4,011,087 to Du Pont (Mar. 8, 1977) PA0 U.S. Pat. No. 4,725,710 to Ford Motor Company (Feb. 16, 1988) PA0 U.S. Pat. No. 4,057,671 to PPG (Nov. 8, 1977) PA0 U.S. Pat. No. 4,940,884 to PPG Industries (Jul. 10, 1990) PA0 U.S. Pat. No. 4,820,902 to PPG Industries (Apr. 11, 1989) PA0 U.S. Pat. No. 5,414,240 to PPG Industries (May 9, 1995) PA0 U.S. Pat. No. 4,543,466 to Ford Motor Company (Sep. 24, 1985) PA0 U.S. Pat. No. 4,976,503 to Monsanto Company (Dec. 11, 1990) PA0 U.S. Pat. No. 4,943,140 to Monsanto Company (Jul. 24, 1990)
Covers using dual leads connecting to a bus bar. This provides at least partial insurance against electrical failure of one of the leads. PA1 A transparent heater is described in which the heating element (i.e. a transparent conductive film) is deposited by a dry process (e.g. sputtering, ion plating, CVD, etc.) and the electrodes are deposited by a wet plating method. PA1 A colored layer (e.g. ceramic color print) is applied at the peripheral portion of an electrically heated windshield. The bus bar is formed on that colored layer. PA1 A braided composite lead for an electrically heated insulated glass unit has the spaces between the braided wires filled with a substantially moisture impervious material to minimize water vapor penetration into the insulating air space. PA1 An electrically heated laminated aircraft glazing in disclosed in which the bus bar is covered with a thin layer of poly(isobutylene) resin containing carbon black. A second similar poly(isobutylene) resin layer is disposed over an extended portion of one glass sheet. In both cases the resin layer is next to a plastic interlayer. The purpose of these layers is to prevent delamination and cold chipping of the glazing with subsequent moisture ingress and bus bar failure. PA1 A laminated electrically heated windshield is shown having accordion-folded bus bars making surface to surface contact with a transparent electroconductive film. This shape of the bus bar is to insure sliding contact between the bus bars and the film when the glass panel and the bus bar expand at different rates. PA1 A heatable windshield is disclosed having a notch for inserting a circuit board which connects to the internal heating element. A means for sealing the notch during lamination is also shown. The connection to the conductive coating is completely sealed so that moisture is unable to penetrate the laminated construction. PA1 A metallizing composition is described which is useful for printing metal patterns on glass substrates to make heaters. The compositions contain silver particles covered with silver halide. These compositions are more resistant to sulfur attack. They are useful for making demisters on automobile windshields. PA1 An electrically heated vision unit is shown in which the bus bar has a wide portion and a narrow portion. The wide portion is more conductive and thus the zone between the two wide regions conducts more current and is selectively heated. PA1 A transparent electrically conductive window with self-soldering bus bars is set forth. The bus bars contain particles of finely divided electroconductive metal (e.g. silver) dispersed with a metal alloy having a fusion point between 70.degree. C. and 150.degree. C. The soldering happens during the lamination cycle. PA1 A heated transparency is provided with greater reliability by providing dual electrical connections to the bus bar that is opposite the edge which is attached to the power source. PA1 A pair of electroconductive extensions extend from the a bus bar in a heated windshield. The extensions reduce power loss in the bus bar and provide redundancy should one of the extensions fail. PA1 An electrically heatable windshield with a hidden bus bar configuration is described. A border of opaque ceramic material, preferably a lead borosilicate enamel, is bonded to an interior surface of the transparency along its periphery. Opposing electroconductive bus bars, preferably a low frit content silver-containing ceramic material, are bonded to the ceramic material so that the entire inner edge of the bus bar overlays a portion of the ceramic material. PA1 A trapezoidal-shaped electrically heated windshield has, at top and bottom edges, bus bars of uniform conductivity through out their length. The upper bus bar runs the full length of the conductive coating which makes up the windshield heating element. The lower contact is symmetrically located along the lower edge of the conductive coating and has a length equal to the sum of the length of the upper bus bar and generally about one-half the difference between the entire effective length of the lower edge coating minus the length of the line of contact of the upper bus bar. PA1 A gradient band is described in an electrically heated windshield. It is formed by thickening one of the metal layers in the transparent conductor (i.e. an interference filter). Plural bus bars run substantially parallel to the gradient band at the top and bottom of a windshield. PA1 A gradient band for automobile windshields is described which is a continuation of one of the metal layer in an interference coating which is generally provided for solar control or electrical heating. Also included is a layer (e.g. NiCr) added to suppress luminous reflection within the vehicle.